1. Field of the Invention
This invention involves the fabrication of optical fiber waveguides.
2. Description of the Prior Art
Two basic properties are of primary interest in the design of optical fiber waveguides for use in long distance transmission. The first involves the loss properties of the fiber. Clearly, the greater the loss in signal strength as the signal traverses the fiber, the greater the need for repeaters and the less commercially viable the resulting transmission system. Current technology is capable of yielding fibers of loss less than 10dB/km, which makes repeaterless transmission feasible for distances as great as 5km.
The second property of interest in the design of an optical fiber involves pulse dispersion. When the optical signal is transmitted in the form of optical pulses, the width of the pulse must maintain a value reasonably close to its initial value in order to prevent overlap between the various pulses and hence reduction in the high bandwidth capabilities of the system. The significance of the present invention can be more realistically evaluated with a greater understanding of pulse dispersion and the means used for reducing its undesirable effects.
Pulse dispersion originates in at least two distinct physical processes. The first is referred to as "material dispersion" and is related to the well-known dependence of the velocity of light in a given medium on the frequency of light being transmitted. A pulse of light which is not purely monochromatic will broaden as it is transmitted through the waveguide material due to the different velocities of the various frequency components of the light which comprise the pulse.
The effects of material dispersion are usually over-shadowed by a second problem referred to as "mode dispersion". The light transmitted through an optical fiber waveguide can be considered as propagating in any one of a large number of modes. Each mode may be thought to be associated with a particular path which a light ray traverses in propagating through the fiber. One mode or path proceeds directly down the center of the fiber. The paths associated with other modes involve reflection off the walls of the fiber any number of times, depending upon the particular mode. Clearly, each mode has associated with it a particular path length. The central mode has the shortest path length. The modes corresponding to reflected paths have longer path lengths. In a single composition fiber the amount of time required for a given signal pulse to traverse the fiber in a given mode will vary directly with the path length associated with the given mode. Hence a given pulse, which may be transmitted in a multitude of modes, will be broadened during its traversal of the waveguide because of the different traversal times associated with the different modes, i.e., those parts of the pulse propagating in the short distance modes will arrive at the far end of the fiber earlier than those parts of the pulse propagating in the long distance modes. Technically the modes do not have a path length. This terminology refers to the length of the ray path associated with the mode. Clearly, a single mode fiber will not display this pulse dispersion phenomenon, but multimode fibers have important applications and the pulse dispersion problem must be solved before many of these applications can be effectively realized.
Reduction in pulse dispersion may be realized in a fiber which has a radially graded index of refraction, with a maximum index at the fiber center and a minimum at the fiber core perimeter. Such fibers are discussed, for example, in U.S. Pat. No. 3,826,560 issued July 30, 1974. The reduction of pulse dispersion in such fibers is in part related to the fact that the velocity of light is inversely proportional to the index of refraction of the material through which the light is propagated. Hence, in a radially graded fiber of the type just described, the velocity of light will be higher near the walls of the fiber and lower at the fiber center. Since the long distance modes are predominately located near the fiber perimeter, the radial gradation will tend to compensate for the pulse dispersion associated with the different path lengths of the various modes. Under such circumstances the traversal times associated with the various modes will be more nearly equal and the pulse dispersion will be minimized.
In an article by S. D. Personik published in Volume 50 of the Bell System Technical Journal at p. 843, an alternative technique was suggested for reducing pulse dispersion. Personik suggested that while normal pulse dispersion increases the width of a given pulse proportionately to the length of the fiber, a fiber which is fabricated so as to enhance conversion between the various propagating modes will result in pulse dispersion which increases the width of the pulse proportionately only to the square root of the length of the fiber. Such mode conversion may be induced by fabricating the fiber with periodic longitudinal variations in its optical properties such as index of refraction, or in its physical properties such as the diameter of the transmitting core of the fiber.
The diameter of an optical waveguide has been controlled during the drawing process in prior art processes. For example, in U.S. Pat. No. 3,865,564 issued to R. E. Jaeger on Feb. 11, 1975 at column 7, line 12, a technique is described for monitoring the diameter of the fiber and changing the drawing parameters in response to variations in the fiber diameter, in order to attain a fiber of approximately constant diameter. In FIG. 4 of the Jaeger patent, it is clear that the monitoring device is placed a significant distance from the heating apparatus and, consequently, it is apparent that this feedback mechanism cannot, and was not meant to, fabricate fibers with low period diameter variations necessary for efficient mode conversion.
U.S. Pat. No. 3,912,478 issued to H. M. Presby on Oct. 14, 1975 describes a technique for fabricating a fiber with diameter variations of sufficient periodicity to enhance mode conversion. In this technique the fiber is periodically cooled thereby producing the requisite diameter variations.